Such membranes are used predominantly in pressure or force sensors. They seal the sensor interior with respect to the outside world and must withstand the pressure difference between both sides. Usually membranes are axisymmetrical. They comprise an elastic region which is delimited by an outer edge and an inner edge. The outer edge is connected in a pressure-resistant manner to a sensor housing whilst the inner edge transitions in a pressure-resistant manner into a movable plunger, the deflection of which can be detected by a measuring element in the sensor housing.
Fundamentally different types of membranes are used, for example, in drums, loudspeakers, microphones or in other technical application. These consist of flat or embossed tiles or films. They are usually not airtight and do not withstand pressures of several bar. They are frequently used in connection with sound waves and are therefore not exposed to any large pressure loadings. Such membranes having diameters of several centimeters or even decimeters must merely allow large deflections. This is achieved, for example, by a plurality of corrugations in the elastic region which lead to good elasticity. However, the material thickness in these regions is always constant.
Likewise, fundamentally different sensor membranes are known from oil-filled pressure sensors. Such membranes are frequently made of soft metals and have corrugations on their surface. Unlike the membranes described here, however, they do not need to withstand any pressure differences since a counter-pressure at the same level as the pressure produced always acts on the inner side of the membrane.
The material thickness of membranes of many pressure or force sensors has a minimum approximately centrally in the elastic region and increases steadily on both sides of the minimum. This type of membrane is known, for example, in CH 670310. The elastic region is concave on both sides when viewed in cross-section, other applications are plano-concave.
As a result of this reduced material thickness of the membrane, which is usually formed of metal, the membrane becomes elastic. The strength of the membrane diminishes with decreasing minimal material thickness whilst the elasticity increases in this case. The opposite is the case in the event of an increase of the minimal material thickness. As a result, load-bearing capacity and elasticity are coupled to one another. Therefore, for example, if the strength is to be increased by a greater material thickness, the elasticity of the membrane will be reduced at the same time, which leads to an increased installation sensitivity.
FIGS. 1 and 2 each show a schematic diagram of a sensor 1 according to the prior art in section. The sensor 1 comprises in each case a housing 6 in which a measuring element 8 is located. The housing 6 is sealed with a membrane 2 which is exposed to a force or a pressure from the outside during a measurement. The membrane 2 is usually round. At the centre it has a plunger 7 whose deflection can be detected by the measuring element 8.
In FIG. 1, the measuring element 8 is a body made of piezoelectric or piezoresistive material which is supported directly or indirectly at the back on a housing 6 and which can measure the force acting upon it from the membrane 2.
In FIG. 2, the measuring element 8 is an optical fibre which can measure the deflection of the membrane 2 whereby light is emitted from it towards the membrane 2 and reflected thereon, received by the fibre again and then evaluated in an evaluation system on the basis of the distance covered.
The membrane 2 comprises in each case an elastic region 3 which is delimited by an outer edge 4 and an inner edge 5. The outer edge 4 is connected in a pressure-resistant manner to the sensor housing 6 and the inner edge 5 transitions in a pressure-resistant manner into the movable plunger 7. During a measurement this elastic region 3 must withstand the loads acting from outside on the membrane 2. Accordingly, under the loading provided it must neither tear nor undergo plastic deformation and should also cause the lowest possible secondary force. The membrane 2 shown here is configured to be concave on both sides in the elastic region when viewed in cross section having a material thickness d which is minimal at the centre and increases on both sides. Plano-concave configurations are also known.
The plunger can alternatively also be configured to be annular, as in CH 670310 whereby the membrane has an elastic region respectively inside and outside the ring. In this case, the central region of the membrane is connected to the housing in a pressure-resistant manner in the same way as the outer edge.
Other membranes for pressure or force sensors are uniformly thin-walled in the elastic region with constant material thickness. At the same time, this thin region can be flat, curved or multiply curved. Such membranes are inexpensive and are used in many applications. However, since their quality relating to the relationship between strength and elasticity is much lower than the quality of membranes having a thinnest point from which the membrane thickness increases on both sides, such membranes are disregarded.
Pressure sensors having various membrane forms are presented in US 2004/0231425. The material thickness in these membranes is usually uniformly thick or has a maximum centrally in the elastic region. Such membranes are predominantly used in high-temperature applications. The aim of these membranes is to reduce measurement errors as a result of material elongations.
Another membrane structure is known from EP 649011 which has heat-compensating effects in pressure transducers. This is characterised by two elastic, plano-concave-shaped regions. Between these regions the membrane is configured to have uniform material thickness and is slightly set back with respect to the plunger.
Such membranes have proved to be weak when the membrane is exposed to high pressures or forces since there is a cavity behind the membrane in the sensor, which is not exposed to pressure. In high-pressure applications or exposure to high forces, rupture of the membranes therefore occurs repeatedly in such membranes.